1. Field of the Invention
The present invention relates generally to brushless motors and, more specifically, to a flat type brushless motor of the type that includes a plurality of spiral coils disposed on a stator yoke; a permanent magnet disposed on a rotor at a position thereof opposing the spiral coils; and a detecting device for detecting the phase of rotation of the rotor so that an electric current is supplied to the coils while being sequentially switched over therebetween at a predetermined timing, thereby generating drive torque.
2. Related Background Art
A capstan motor which is commonly used with a magnetic recording/reproducing apparatus or the like is illustrated in FIG. 1 as one typical example of a conventional type of brushless motor. A stator yoke 1 is attached to a housing 2, and a plurality of spiral coils 3 are disposed on the stator yoke 1 around the circumference thereof. A rotor yoke 4 is supported via a bush 6 on a rotating shaft 5, and a multipolar magnet 7 is attached to the rotor yoke 4 at the position thereof opposing the spiral coils 3. An FG magnet 8 which is magnetized at a fine pitch is disposed around the outer periphery of the rotor yoke 4, and a magnetism detecting device 9 is attached to the stator yoke 1. In general, the magnetism detecting device 9 is constituted by a semiconductor magnetic resistance device for detecting variations in magnetic resistance. The magnetism detecting device 9 detects the state of rotation of the rotating shaft 5 from variations in the magnetic field caused by the magnet 8.
A Hall element 10 is fixed to the stator yoke 1, and the Hall element 10 can be used to detect the phase of the multipolar magnet 7 while the rotor yoke 4 is rotating about the axis of the shaft 5.
A ball bearing 11 and a metal bearing 12 cooperate with each other to support the rotating shaft 5 for rotation about its axis with respect to the housing 2.
In the brushless motor having the above-described construction, torque is produced by supplying an electric current to the coils 3 while the current is sequentially being switched over among the coils 3 at a predetermined timing based on the output of the Hall element 10, thereby driving the rotor yoke 4. In a case where such a motor has three phases, a composite torque is produced in a manner such as that shown in FIG. 2.
More specifically, if n represents the number of magnetic poles of the multipolar magnet 7, the distributions of magnetic flux density in the coils of the respective three phases varies in the form of sine waves, with (a) a first phase, (b) a second phase, and (c) a third phase shifted one from another as shown in FIG. 2A, since the coils 3 corresponding to the respective three phases are disposed on the stator yoke 1, and are the coils 3 out of phase with respect to the magnetic poles by an electric angle of 120.degree.. A current is sequentially supplied to the coils 3 of the three phases in the positive and negative directions at the timing shown by wave forms a, b, and c in FIG. 2B. Therefore, torque is generated in correspondence with each individual phase as shown in a wave form d of FIG. 2C, and the resultant composite torque assumes a wave form e in FIG. 2C. Thus torque ripples are formed.
Such torque ripples cause non-uniform revolution. If T represents the magnitude of the torque ripples, N the rotational speed of the rotating shaft 5, J the inertia force, and .DELTA.N the degree of non-uniformity of rotation, the relationship represented by the following expression is established: ##EQU1## Therefore, in particular, use of a capstan whose rotational speed N is low increases the degree of non-uniform rotation .DELTA.N due to the torque ripple T. This results in a problem in that magnetic recording/reproducing apparatus will experience wow and flutter at low frequencies as in sound signals and jitters as in video signals. To cope with this problem, inertia force has in the past been increased. However, adoption of this measure is retrogression in relation to the desirable tendency to seek a reduction, in the total weight of the apparatus.